Emptying time of a tank filled up with explosive paste. Comparison between experimental measurements and predictions based on rheological characterization of the paste
Résumé
One industrial process used by Nexter Munitions to manufacture pyrotechnical materials consists in preparing an emulsion of wax in TNT (2,4,6-trinitrotoluene) and adding Aluminium and ONTA (3-nitro-1,2,4-triazole-5-one) particles. When the suspension is homogeneous, it is allowed to flow by gravity through a pipe located at the bottom of the tank and to fill up a shell body. The suspension is characterized by a solid volume fraction of 53.4%, which leads to high viscosities. In some circumstances, the emptying time is prohibitively long and the economic profitability is reduced. This study has been performed to make the emptying time lower with the constraint of unchanged volume fractions and grains mean diameter. So, we investigated the influence of the grain size distribution on the suspension viscosity. Different samples of Aluminium and ONTA have been used, with rather small differences in grain size distributions. The suspensions have been prepared in the industrial tank and the flow cast times measured. It has been observed that they differ by one order of magnitude. To avoid situations with too high emptying times, a procedure has been implemented to make prior characterization of the suspension rheology. Because of particles sedimentation and emulsion destabilisation, the classical Couette rheometer is not adapted. So, we designed and built a small size tank (113 cm3), where the suspension is continuously stirred and kept homogeneous. The measurement of the torque and rotational speed together with the use of the Couette analogy allowed us to observe an Ostwald fluid behaviour (flow consistency index k, flow behaviour index n). To gain in prediction, we established a correlation between the measured (k, n) values and the grain size distributions. We characterized each suspension by the ratio of to , where is the solid volume fraction (imposed by the commercial specifications) and is the maximum packing fraction. Because of the strong analogy between concrete and energetic paste, we chose the widely used De Larrard model to compute . A linear dependance between the ratio and the indices k and n was observed. The second step was to provide an analytical expression for the flow cast time of a power-law suspension from a tank with a given geometry. Considering the large difference between the industrial inner tank diameter and the evacuation pipe diameter, we assumed that all the pressure drop was located in the evacuation pipe. Then, extending the Hagen-Poiseuille equation to Ostwald fluid, we were able to predict the emptying time with the knowledge of k and n. Experimental and predicted emptying time are in very good agreement. This work helped the industrial manufacturer to divide the emptying time by a factor 12.
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